The human body is fantastically complex  and fantastically capable. It is dependent on smaller, simpler units that each serve specific purposes. From the simplest to the most complex, they are:

The basic building block of every living thing is the cell. Cells themselves are extremely complex, come in many different shapes and sizes, and serve countless different functions. Most cells are microscopic.

A group of similar cells gathered together is called a tissue. Tissues, which may be visible to the naked eye, include bone,muscle, fat, and skin.

Different kinds of tissue working together in the same place may form an organ. Organs, like the heart,lungs, eyes, and brain, perform specific tasks necessary for the body's survival.

Several organs can work together in a system. The organs of a system may be close together, or spread across the body. Some examples are the digestive, reproductive,respiratory,excretory, and nervous systems.

Some kinds of tissue may be found in more than one system. Muscle tissue, for example, is part of the muscular, respiratory, and circulatory systems. Some organs, like the pancreas (which is part of the digestive and endocrine systems)pull double duty.